[PATCH] net/netvsc: fix event buffer leak on realloc failure

[Chen Zhao]

Hi,

The AI review reports a double-free and a dangling event_buf:
https://mails.dpdk.org/archives/test-report/2026-October/1069492.html

Both findings are false positives:

1. On successful reallocation, goto retry skips rte_free(old_buf).
   The retry label is outside the ENOBUFS block. Each subsequent entry
   into that block reinitializes old_buf from the current event_buf,
   so a later allocation failure frees the current allocation, not an
   address retained from a previous successful reallocation.

2. On failure, rte_realloc() returns NULL without freeing the old buffer.
   That NULL return value has already been assigned to rxq->event_buf
   before rte_free(old_buf) is reached. Thus event_buf is already NULL;
   no additional NULL assignment is needed.

The patch preserves the existing reset-and-retry behavior while freeing
the old allocation on failure.

Thanks,
Chen